In the making of glass, glassmaking materials are provided into a glassmelting furnace and melted into molten glass which is then poured into molds to produce products such as, for example, glass bottles. The glassmaking materials include batch oxidizers such as salt cake (calcium sulfate, CaSO.sub.4) and niter (sodium nitrate, NaNO.sub.3, and potassium nitrate, KNO.sub.3) in order to control the redox state of the glass.
The glassmaking materials are melted in the furnace by heat provided by the combustion of fuel and oxidant. Water vapor resulting from the combustion reacts with alkali oxides in the molten glass to form alkali hydroxides which vaporize out from the molten glass. These alkali hydroxides, such as sodium hydroxide, NaOH, react with furnace refractory walls and cause refractory corrosion, and, further react in the flue passage after the furnace with sulfur dioxide, SO.sub.2, and oxygen to form sodium sulfate, Na.sub.2 SO.sub.4, and other sulfate and sulfite compounds which form particulates and often require expensive electrostatic precipitators or baghouses to ensure that they are not emitted to the atmosphere.
Accelerated corrosion is experienced in "super structure" refractory bricks in glassmelting furnaces that are converted to oxy-fuel firing. In particular, severe loss of silica crown is observed in some glassmelting furnaces such as in glass melting for TV panels. It is generally believed that the main cause of the accelerated corrosion is the higher concentrations of volatile alkali species, such as sodium hydroxide (NaOH) and potassium hydroxide (KOH), under oxy-fuel firing.
In oxy-fuel firing nitrogen contained in the combustion air is largely removed and the volume of the combustion products is typically reduced to 1/3 to 1/4 of that of the conventional air firing. Thus the concentrations of alkali species would increase three to four times as compared to the, same amount of volatile alkali species generated in conventional air firing.
Accelerated corrosion shortens the furnace life and results in costly furnace repairs. In addition, corrosion increases glass defects in some glass tanks due to dripping of slag into the glass bath. Corrosion resistant refractory bricks such as alumina and alumina-zirconia-slica (AZS) bricks have been used to alleviate this corrosion. For example, AZS is often used for side walls and flue port walls, of glass furnaces, to control the corrosion problems. Silica bricks are the most widely used refractory material for the crown of furnaces because it is lighter, less conductive and substantially less expensive than alumina and AZS bricks. Also, there is a concern for increasing glass defects caused by zirconia "refractory stones" when AZS is used for the crown. When silica is used as the material that makes up the crown of the furnace, corrosion, which causes dripping of slag into the glass bath, does not necessarily result in glass defects. This is because silica is the main composition of glass.
In order to reduce volatilization of alkali species from glass and batch surfaces hot spots on the glass and batch surfaces and high gas velocities near the surfaces should be avoided. This is accomplished by using low momentum oxy-fuel flames placed at least 12 inches above the surface of the glass bath. A "low momentum oxy-fuel flame" is defined as a flame formed by reacting a fuel and an oxidant containing at least 30% O.sub.2 which has a moementum-averaged velocity less than 200 ft/sec, preferably less than 100 ft/sec, at the exit plane of the gas exit port of the burner, such as, for example, the oxy-fuel burner in U.S. Pat. No.5,449,286, incorporated herein by reference. The flame is directed substantially horizontally to avoid the flame impingement on the batch and glass surfaces.
It would be very desirable to provide a glassmelting method wherein silica bricks can be used to line the crown of the furnace and wherein volatilization of alkali species is reduced to minimize corrosion of the crown.